Aqueous Swelling of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in the Presence of Ionic Surfactants

Superhydrophilic polyzwitterionic brushes resist fouling, but free ions may screen zwitterion charges and alter brush hydration. This work examines the effect of ionic surfactants on polyzwitterionic brush swelling. In situ ellipsometry shows that the swelling of poly­[2-(methacryloyloxy)­ethyl­dimethyl-(3-sulfopropyl)­ammonium hydroxide] (PMEDSAH) brushes depends on surfactant charge and concentration as well as film thickness. Solutions containing ≥6 mM sodium dodecyl sulfate (SDS) increase the swollen thicknesses of PMEDSAH brushes 2- to 9- fold with respect to thicknesses in water, and increases in swelling are especially high (6- to 9- fold) for thin films. Surfactant adsorption likely breaks ionic cross-links in brushes to enhance swelling, and immersion of brushes in 500 mM NaCl also leads to extensive swelling. Fitting of in situ ellipsometry data suggests that highly swollen films consist of a relatively dense, collapsed base layer covered by dilute brushes that contain about 98% water. At 10 or 20 mM surfactant concentrations, dodecyltrimethyl­ammonium bromide (DTAB) yields much smaller increases in swelling than SDS, presumably because of the hydrophobicity of DTAB. Fluxes through PMEDSAH-modified microfiltration membranes are higher with 2 mM DTAB than with 6 mM SDS, consistent with the higher swelling of thin PMEDSAH brushes in the SDS solution.